1. Field of the Invention
The present invention relates generally to navigational signal receivers, and more particularly, to Numerical Controlled Oscillators (NCOs) for navigational signal receivers.
2. Description of the Related Art
Satellite-based radio navigation systems have become widely adopted in many commercial and military applications. Exemplary systems in operation or development include the NAVigation Satellite Timing and Ranging Global Positioning System (NAVSTAR GPS), the Global'naya Navigationnaya Sputnikovaya Sistema (GLONASS), a European satellite navigation system called GALILEO, the wide area augmentation system (WAAS), and the local area augmentation system (LAAS). These systems permit a user with an appropriate direct sequence spread spectrum (DSSS) signal receiver to determine his or her position with respect to the Earth. Direct Sequence Spread Spectrum is a modulation technique where a pseudorandom noise sequence directly phase modulates a data-modulated carrier. The DSSS signal has a noise-like spectrum and appears to be noise to all but the intended receiver.
As an example, the GPS constellation has 24 operational satellites. These satellites are positioned in six different orbital planes such that at any time a minimum of six satellites are visible to any user on the surface of the Earth, except in the polar region. The satellites operate in near circular 20,200 kilometers (about 12,000 miles) orbits at an inclination angle of 55 degrees and with approximately a 12-hour period.
Each satellite contains at least one atomic clock and transmits a navigation message that contains an accurate system time and its orbital position referenced to the atomic clock. The navigation message also contains clock behavior, status messages, and correction data such as ionospheric delay, time offset, etc. An almanac that gives the approximate data for each active satellite is also provided.
Each satellite transmits on two L-band frequencies: L1=1575.42 MHz and L2=1227.6 MHz. Three sets of pseudorandom noise (PRN or PN) ranging codes are in use: the coarse/acquisition (C/A) code, the precision (P) code, and the Y-code.
The C/A code set, also known as Gold code, has a 1.023 MHz chip rate. In spread spectrum technology, the term “chip” refers to a single bit of a pseudorandom sequence (PN-sequence) and the term “chip rate” refers to the rate at which bits of a PN-sequence are shifted. The Gold code therefore has a length of 1023 chips. The term “code” refers to the binary bit stream (the pseudorandom sequence) used to spread a signal over a wide range of frequencies for transmission. This spreading improves the accuracy of position estimation. Other advantages include interference rejection and low spectral power density, i.e., the power level at a given frequency.
A correlator at a receiver despreads this signal to the original data bandwidth by correlating it with a locally generated PN-sequence identical to and in synchronization with the PN-sequence used to spread the carrier at the radio transmitter, e.g., a GPS satellite vehicle (SV). Typically, this despreading occurs after the signal received at the antenna has been amplified and down-converted to a suitable low carrier frequency, also known as the intermediate frequency (IF). The hardware section associated with the amplification, down-conversion, and analog-to-digital conversion (ADC) is called the radio frequency (RF) stage. The other section, which processes the RF stage output and generates the position, velocity, and time information, is called the baseband (BB) stage.
There are two Numerically Controlled Oscillators (NCOs) in GPS baseband. One of the NCOs is used to generate the IF carrier frequency while the other is used to generate the code frequency, which corresponds to the PN code rate. The former is denoted as carrier NCO while the later is denoted as code NCO. The sampling rate at the BB stage can be any multiple of the PN code rate. A minimum of two samples per chip (bit) is needed, which results in a minimum sampling rate of 2.046 MHz. The sampled signals are then made available in two channels, one in-phase (I) and the other quadrature-phase (Q). The resulting signals are then correlated with the locally generated PN code. The local code generator is driven by a code NCO. The result of the correlation is sent to a processor and further processed to determine the code frequency and carrier frequency, as well as code phase and carrier phase. The processor sends a control signal to the code NCO and the carrier NCO so that they are in alignment with the input signal. Usually, this correction is not done every millisecond. It depends on the periods of the carrier frequency tracking loop and delay lock loop. In some cases, the correction period can be up to several seconds. Thus some average correction is applied to multiple samples. When the incoming signal is aligned with the locally generated PN code and carrier, the data bits in the signal can be extracted. The extracted data are used in computing the satellite position and hence the receiver's position, velocity, etc.
It is necessary to acquire the satellite signal in order to determine the pseudorange or approximate distance to the navigation satellite from the receiver and to extract the navigation data. The Direct Sequence Spread Spectrum (DSSS) signal employed requires a perfect correlation of the received signal with a locally generated PN code in order to acquire the signal. Additionally, the local carrier frequency should be sufficiently close to the received signal frequency, in which the closeness depends upon the intended length of integration or correlation. In the exemplary case of GPS, the first or short time integration is done over a length of 1023 chips with an associated time duration of 1 ms. This requires a residual carrier frequency of less than 500 Hz. Any increase in this residual frequency will result in some of the samples within the correlation or integration length being phase reversed with a negative contribution to the integration value. This decreased integration value results in the receiver not being able to acquire the signal. This problem becomes more pronounced as the coherent integration length is increased. In an exemplary case where the integration length is increased to say 2 ms the residual frequency needs to be less than 250 Hz. Thus the residual frequency puts a constraint on the coherent integration length. In such cases non-coherent integration in which small length coherent integration powers are considered is used. However, this is an inefficient method and so coherent integration is usually preferred.
The signal becomes weak due to receiver operation in indoor conditions or when the signals are blocked as in the case of foliage or urban canyon. A lengthy coherent integration, sometimes extending up to several hundred milliseconds, is needed to acquire the weak signal. In additions to this, several sequential correct signal confirmation stages may be required. The residue frequency error between the locally generated frequency and incoming IF signal from the RF module during this integration interval should be small and should not reduce the acquisition sensitivity. As an example, a Fast Fourier Transform (FFT) with downsampling can be used for long time coherent integration. In a case of, e.g., a 5120 ms length integration with a downsampling of 20 times, the resulting FFT points will be 256 with a corresponding frequency resolution of 0.2 Hz. That means that if the frequency change during the integration is more than 0.1 Hz, then the signal power will be dispersed to two or more frequency bins. This leads to the decrease of peak power and makes the acquisition or tracking sensitivity lower. Thus when the integration is long, the frequency change during the integration must be considered even though the receiver is static. The Doppler frequency change due to the satellite dynamics alone has an average value of 0.5 Hz/Sec with a maximum of 1 Hz/sec.
As already discussed earlier, the phase and frequency of the samples are corrected by the carrier NCO and code NCO with the same correction factor for a set of values. This correction factor being their average value does not correct each sample with correct phase and frequency values. Usually, the integration is done in two stages: separately computing the short or 1 ms integration and then integrating these short length integrations over the desired length with necessary phase correction for each of the short length integrations. This is how a typical long integration is presently carried out. An exemplary case has been explained in the U.S. patent application Ser. No. 11/123,861 filed May 6, 2005, which is incorporated herein by reference.
However, compensating the set of samples with their average phase does not remove the phase involved with each of the samples. In a set, such compensation may correctly compensate the center sample while leaving the remaining samples with some uncompensated phase error. This phase error increases as the sample position is farther away from the center. U.S. Pat. Nos. 5,365,182 and 5,192,957 disclose changing the phase compensation according to the Doppler frequency or the rate of change of distance between the receiver and the satellite, but do not attempt to compensate each sample individually. An individual phase compensation of the samples based on an estimation of the phase improves the associated long coherent integration performance.
Clearly, there is a need for better phase and frequency compensation of each samples in order to compute a long coherent integration.